The aircraft had just completed a chemical application run of approximately 1/2 mile in length from north to south. The pilot attempted to turn the helicopter from a heading of south, through a heading of east, to the north or west. During the turn, the aircraft descended in uncontrolled flight until it collided with the terrain. The aircraft had been refueled and loaded with about 350 gallons of chemicals about 5 minutes before the accident. No mechanical deficiencies could be found. The wind at the accident site was reported from the south. During the turn from the south to the north, the aircraft passed through a heading where the wind would be off the right rear of the aircraft. At this particular heading, maximum power would be required to sustain flight. In consideration of the aircraft's weight; direction of turn; wind direction; and witness reports, the aircraft experienced settling with power. During the hover maneuver, the aircraft was at a relatively low altitude which prohibited recovery.